In recent years, for the purpose of expanding the coverage areas of base stations, relay transmission techniques have been studied in which a relay apparatus (RN: Relay Node) is installed between a base station and terminals, and communication is performed between the base station and the mobile stations via the relay apparatus. Multihop communication is regarded as a promising means for providing wideband transmission to areas outside macrocell service areas particularly in cellular environments. In the multihop communication, a plurality of relay apparatuses are connected in series, and communication is performed between a base station and terminals.
Furthermore, in consideration of expansion of use of radio waves not only between, e.g., mobile terminals, but also between various types of apparatuses in the future, a shortage of usable frequencies inevitably occurs. Thus, further enhancement in use efficiency of frequencies or expansion of the usable frequencies to a high-frequency band becomes essential. Multihop communication is a promising communication means also as a means for meeting such demands for, e.g., the enhancement in use efficiency of frequencies and the expansion to a high frequency band.
More specifically, when a communication distance between a base station and a terminal is the same, a communication distance between the apparatuses in two-hop communication (multihop communication) is a half (½) of a communication distance between the apparatuses in single-hop communication. Here, in consideration of a case where received signal power is inversely proportional to the square of the distance, when the same received signal power is provided in two-hop communication and single-hop communication, the transmission power in two-hop communication can be only ¼ (=(½)2) of the transmission power in the single-hop communication. Furthermore, a frequency reuse distance can be reduced to a half of that in single-hop communication in two-hop communication, and thus, a simultaneous communication density for the same frequency is approximately four times (=22) that in single-hop communication. However, the transmission timing needs to be divided into two in two-hop communication. Thus, the throughput in this case becomes a half of that in single-hop communication in end-to-end communication (between a base station and a terminal). However, in two-hop communication, an area spectral efficiency, which is provided by the product of a throughput and a simultaneous communication density, is twice (=(½)×4) that in single-hop communication. In other words, employment of multihop communication (here, two-hop communication) enables enhancement in frequency use efficiency as well as enhancement in throughput.
Furthermore, signals have high linearity and the propagation loss (pathloss) is large in a high-frequency band (high carrier frequency). Thus, when the same transmission power as that in a low-frequency band (low carrier frequency) is used, the received signal power decreases. However, in multihop communication, the propagation loss (pathloss) per hop can be decreased by a further decrease in distance between transmitting and receiving apparatuses. In other words, employment of multihop communication enables flexible support for expansion of usable frequencies to a high frequency band according to the number of hops.
As an example of multihop communication, a conventional technique is under study, in which a plurality of relay apparatuses perform communication between a base station and terminals using the same fixed frequency (see PLT 1 or the like), for example.